1. Field of the Invention
The present invention relates to an outboard motor having an improved structure of a cooling water passage and an associated structure.
2. Discussion of the Background
Engines of outboard motors include an engine, generally of water-cooling type, having a cylinder block in which a plurality of cylinders are arranged in a vertical direction. In a cylinder head connected to the cylinder block, intake ports and exhaust ports are formed for the respective cylinders. Exhaust gas discharged from the exhaust ports is collected into a common exhaust passage and then discharged out from the outboard motor.
Since the exhaust passage is heated to a very high temperature by the exhaust gas, a cooling water passage is formed around the exhaust passage to cool the exhaust passage by means of cooling water. Such a cooling water passage is provided for members of the engine, such as cylinder block, an engine holder and an oil pan, through which the exhaust passage passes.
However, in a conventional structure, since fastening bolts for connecting the engine holder and the oil pan are disposed only around the members mentioned above, a pressure acting on a juncture (mating) surface between the engine holder and the oil pan near the central portion of both these members adversely becomes lower than that of a peripheral portion of the juncture surface. As a result, in a case where the cooling water passage is formed nearer to the central portions of the engine holder and the oil pan, if the cooling water pressure becomes high, the cooling water leaks from the central portion of the juncture surface of both the members.
It is possible to arrange the cooling water passage closer to the peripheral portions of the engine holder and the oil pan than to the exhaust passage. However, in such a case, the engine holder can not be formed with a gas collecting port of the exhaust gas and the oil pan can also not be formed with a water checking port, for example.
In the meantime, in the outboard motor equipped with the water-cooled engine, respective parts or elements of the engine are cooled by sea water, lake water, river water or like which is pumped up by a water pump driven by a drive shaft, and after cooling, such water is discharged outward.
At a time when the temperature of the cooling water is low just after the starting of the engine operation, for example, the flow of the cooling water is interrupted by a thermostat valve provided for the engine to thereby assist or promote a warm-up of the engine.
However, during an engine warm-up operation, in spite of the fact that the flow of the cooling water is interrupted by the thermostat valve, the water pump is driven, so that the water pressure in a upstream side of a cooling water passage of the thermostat valve is increased. In order to obviate such defect and protect a seal portion or like of the cooling water passage, in the prior art, there is provided a water pressure valve, for example, as a relief valve, to the cooling water passage at a portion upstream side of the thermostat valve to thereby discharge a portion of the cooling water of which pressure is excessively increased outside the cooling water passage.
Japanese Patent Laid-open Publication No. HEI 8-100658 discloses one example of such water pressure valve, which is disposed inside the engine and Japanese Patent Laid-open Publication No. HEI 10-339163 discloses a structure in which a box-shaped passage casing is mounted to a bottom surface of an oil pan disposed below the engine to guide the cooling water into this casing and a water pressure valve is mounted to this case.
However, these prior art publications provide the following problems or defects.
That is, in the case where the water pressure valve is disposed in the inside portion of the engine, a layout of other equipments or members in the engine will be damaged and, moreover, it will be necessary to locate a specific discharge passage for discharging the cooling water from the water pressure valve, so that the number of the parts will be increased and the entire structure of the engine will be made complicated, thus providing a problem.
Furthermore, in the case where the passage casing is mounted to the bottom surface of the oil pan, the number of parts or elements will be increased and, moreover, since the bottom surface of the oil pan is made flat, lubrication oil will not be sufficiently discharged at a time of exchanging the oil.
Additionally, in the outboard motor shown in the Japanese Patent Laid-open Publication No. HEI 10-339163, an inside portion of a drive shaft housing disposed below the oil pan is divided by a partition wall into an exhaust chamber and a space in which cooling equipments or like are arranged, and the cooling water from the water pressure valve is discharged into this space. However, the wall portion of the drive shaft housing on the side of the exhaust chamber is heated by the exhaust gas and its temperature is increased, so that there is caused a problem of adhesion of calcium, for example, contained in the sea water to the drive shaft housing wall, thus being inconvenient.